Biochar = Cheap & Reliable Carbon Removal

Impact: 3-6% Global Warming Potential (GWP)

Technology Maturity: Scaled/Subsidized

This is the seventh article of a series on the climate technologies shown in my ClimateTech Market Map, with a deeper dive into technical maturity and potential to reduce global warming. Here I'll cover the maturity and potential impact of using biochars as a Carbon Dioxide Removal (CDR) method to offset hard-to-abate greenhouse gas emissions.

Biochar is short for "bio-charcoal": organic matter (such as crop straw) that has been cooked at high temperature without oxygen. Some biochars can persist intact in soil for hundreds (even thousands) of years without degradation, which makes them viable for CDR. Biochar is currently used for many purposes including as an ingredient for lower carbon cement and road asphalt, as a direct charcoal-substitute, and as an agricultural soil amendment.

Incorporating nutrient-primed biochars into agricultural soils improves soil health and crop yields (dramatically in poorer agricultural soils), reduces soil emissions of greenhouse gases (N2O and methane), and in some cases triggers further accumulation of soil organic carbon.

However, there are three challenges for scaling biochar usage for CDR.

1. Most potential biochar feedstocks, such as crop residues and waste poultry litter, have established markets for use as livestock feed, fuels or compost. Guaranteeing feedstock supply at an economic price, particularly when feedstocks have higher economic use as bio-fuels, is a major challenge to scaling biochar.
2. Production costs are at least partly driven by scale and many existing plants are sub-scale, resulting in high biochar prices.
3. Today's biggest biochar end uses do not seem to be for applications like soil amendments that provide CDR. Instead, in some cases biochar production capacity is being used for fuel production ("bio-coal") or optimized to produce fuel co-products (syngas or bio-oil) that replace the use of fossil fuels, but do not sequester carbon. When these uses replace dirty fossil fuels, it can result in greater avoided emissions than when it is used a soil amendment.

It's important to emphasize that even the maximum theoretical biochar CDR capacity is a small fraction of today's global emissions. Researchers have calculated that even 100% diversion of all possible feedstocks in the province would offset ~9% of emissions in Guangdong, China. A back of the envelope calculation for the UK shows that a similar full diversion would offset ~3-6% of total emissions. That 3-6% range is roughly the right range for the global removal potential as well, based on the available research adjusted for feedstock contention.

What are Biochars?

Biochars are made from organic matter that has been pyrolyzed ("cooked") in a low oxygen environment at high temperatures. Feedstocks for biochars can be any organic source with a consistent composition: crop residues (straw, nut hulls, husks), wood waste (brash, sawdust, demolition waste), manures, anaerobic digestate (biogas production waste) or processed food production waste. (Ordinary food waste varies too much in composition to be a suitable feedstock.)

Biochar properties depend on the type of feedstock used, as well as pyrolysis temperature and duration. Multi-decade biochar soil persistence requires a minimum pyrolysis temperature of 400℃, but higher temperatures of 600-800℃ create the most recalcitrant biochars. Biochar yields at this temperature are anywhere from 15-30% of the feedstock mass depending on the feedstock. Biochar is not the only product from the pyrolysis process: various co-products are also produced, depending on how quickly process heat is increased to the temperature target. Slow pyrolysis produces syngas (a mixture of carbon monoxide, ethylene and other volatile organic compounds), whereas fast pyrolysis produces liquid hydrocarbons (bio-oil).

Pyrolysis has a yield/temperature tradeoff, so it may be optimal to use slightly lower temperatures because increased yield outweighs lower persistence. Rodrigues et al. (2023) calculated that the optimal carbon sequestration for their feedstock was produced by pyrolysis at 500-550℃, which yielded a predicted 100 year persistence in soil of 44%.

[Using even lower treatment temperatures in the range 200-300℃ is a process called torrefaction and it produces bio-coal: a low moisture product that can be pelletized or made into briquettes and burned as an a co-fuel in power generation or cement and steel manufacturing.]

Biochar Soil Persistence & Application Rates

Measuring biochars' persistence in soil can be complicated because biochars have 2-3 pools of carbon compounds that degrade at different rates. All biochars have a small pool of carbon that degrades rapidly (within a year) - so short term degradation rates are higher than long term rates. A number of meta analyses have characterized factors that affect soil persistence (expressed as mean residence time - MRT - or as the percentage of original mass persisting at 100 years).

Wang et al. (2016), a meta analysis of ~30 biochar studies found that biochars produced between 400 and 700 ℃ had essentially equivalent persistence, with an estimated 100 years MRT. The authors also found crop-based feedstocks and incorporation into sandy soils were associated with lower persistence.

However, long duration studies have found higher long term persistence rates. Kuzyakov et al. (2014) measured decay rates in years 5-7 of their study and estimated an MRT of 4,000 years. Other studies have found that temperature, water-logging, and soil organic carbon pools can also strongly affect degradation rates.

How much Biochar can soils store?

Field studies generally conclude that the maximum initial application of biochar should be in the range of 10-20 tons per hectare (although some biochar/crop/soil combinations have shown negative yield effects at these levels, so this can't be taken as a consistent maximum). After initial application, effects persist for several years without reapplication. Re-application after 8-10 years is a current loose guideline, but there has been little research to reliably support this.

Assuming, therefore, a levelized application rate of 1 ton per hectare per year, global cropland has the capacity to absorb up to ~5 gigatons of biochar per year, or roughly a third of all global CO2 emissions. This is far in excess of feedstock supply, even if all possible feedstocks were dedicated to biochar, so feedstock supply rather than demand capacity is the limiter on biochar CDR capacity.

Biochar Effects on Soil Emissions

Apart from its sequestration effects, biochar can reduce GHG emissions from cropped soils. Tests of biochar incorporation has found that it reduces N2O emissions by about half in the first year after application, although it is ineffective on acidic soils. Emissions reduction persists in following years but is lower (but the research here is not deep).

Biochar seems to promote the final step in bacterial denitrification which converts soil N2O -> N2, thereby reducing the pool of N2O that can escape from the soil as GHG emissions. (Emissions from pasture soils, where co-denitrification is the dominant N2O pathway do not appear to be affected by biochar.) Researchers have also found methane reductions, but these results are inconsistent, with some studies showing methane increases.

Biochar usually boosts crop yields, but not universally. For tropical soils, in particular many African soils that suffer from low pH, low CEC (nutrient binding capacity) and phosphate-binding chemistries, biochar is an effective remedy that can deliver 25% average crop yield increases - albeit at high application rates (1).

Biochar appears to also slow down the decomposition of soil organic matter - an effect called "priming" - which multiplies its carbon sequestration effect by 40-60%. Biochar has additional environmental benefits including reduced leaching of nitrogen fertilizer and improved soil water capacity. (Biochar can also have negative side effects in some cases - e.g. it tends to interfere with soil pesticides - but I will not cover these here.)

Biochar Production Costs & Supply Chain

Biochar production costs vary widely - depending on scale, the cost and quality of feedstock, co-product revenue (heat/syngas/bio-oil), subsidies, carbon credit prices and whether the production process is optimized for biochar production or co-products. In Europe, the revenue breakdown for biochar producers is approximately 11:5:4 between biochar: carbon credits: co-products (2).

At the low end, biochar can be made in small volumes in hand-dug clay lined pits, or in cheap metal kilns (above). When fed appropriately, these reach high temperatures and can produce hundreds of kilograms of biochar per day. However, they don't produce co-products and are labor intensive. At the other end of the spectrum, there are highly automated scaled plants like the 10,000 ton capacity Airex biocoal/biochar facility which uses wood waste from its co-located forest product business as feedstock.

Production cost estimates for scaled biochar production have a large range. Biochar Zero, a German biochar consultancy, estimates that capital costs alone for smaller scale European biochar plants are ~$4,400 per ton of annual production before subsidies, or roughly half that after subsidies, yielding an annual depreciation expense alone of $110 per ton (20 year lifetime).

Li et al. (2023) reviewed a number of LCA studies that estimated scaled production costs to be in the $450 to $1000 range. Shackley et al. (2011) estimated the range of net production costs for UK biochar at ($222) to $584 per ton based on the potentially large range of feedstock and co-product prices. This puts at least some UK biochar production options in the magic range of $100-200 per ton needed for economically viable CDR.

In the US, Nematian et al. (2021) calculated the cost of small scale (1 ton/day) mobile pyrolyzer production in the California Central Valley at ~$1,400 per ton, which is uneconomic for all biochar use cases.

For China, Deng at al. (2024) estimated production costs for large scale biochar production at ~$170 per ton for crop residue feedstocks, which can be netted against ~$80 of syngas income per ton of biochar produced, for a net production cost of ~$90 per ton. In many regions, we seem to be very close to un-subsidized viability.

However many of these academic costing exercises are done assuming that biochar production facilities are standalone factories with inputs and outputs being shipped in and out. But this is not how biochar facilities are being developed today: instead co-location for feedstock supply and cost-optimization seems to be the dominant strategy in my unscientific scan of providers.

For example, Pacific Biochar (California) adds its pyrolyzer as a co-processor at pre-existing bio-mass electricity plants. Pacific Biochar offers biochar at prices starting at $210 per ton in bulk, which would seem to reflect production costs that are much lower than those calculated in LCA studies. Similarly, the startups doing commercial scale biochar (below) are generally co-locating directly with feedstock supplies at a capacity that matches that supply.

Contention for Feedstock

At a high level, the total volume of organic waste streams produced globally is very large. Crop residues alone are estimated at anywhere between 2.4 billion tons (4) and 5.3 billion tons per year (5). Assuming that 20% of this is reasonably available for biochar production given competing needs, this yields a potential of anywhere from 75-300 million tons per year of biochar from crop residues depending on yield assumptions. Livestock manure and wood waste could add another 25-100 million tons per year. This represents roughly 50kg to 200kg of biochar per hectare of global cropland per year: far less than soil capacity.

The biochar market today is tiny compared to this potential. Combined, the ~150 biochar producers in Europe produced just 53,000 tons of biochar in 2022 at market prices ranging from €300 to €2,000 based on biochar grade (3). This represents less than 0.1% of just the UK's available feedstock.

However, even without looking at the traditional uses for organic waste carbon streams (cooking, compost, and heating) other climate technologies like sustainable aviation fuel, green cement, precision fermentation producers, and insect farming, are also planning to use organic waste streams as part of future production processes.

And even at this early market stage, competition for high quality waste streams has resulted in feedstock price volatility that is causing problems for biochar producers in Europe. Because pyrolysis reactors have to be tuned for specific feedstocks, rapidly converting from one type of feedstock to another as prices change is not possible, so security of feedstock supply is a critical issue.

Climate Optimal Use of Organic Waste Streams

A final, thorny issue for biochar is whether using it for CDR is climate optimal. Some case studies and reviews have calculated that burning organic waste streams for fuel has a greater effect on emissions than using them for biochar, particularly if this displaces fossil fuels.

An in-depth study by Stockholm's District Heating System in 2019 evaluated biochar vs. biofuel production, and concluded that using organic waste streams for fuel provided almost 3x the emissions mitigation as agricultural use because of fossil fuel displacement. However, biochar would provide more emissions mitigation when the power grid is eventually decarbonized (6).

Field et al. (2013) studied a pyrolysis facility in Colorado and concluded that optimizing for fuel production was also more profitable than biochar soil CDR, but only with carbon prices below $50 per ton. Above that price, using biochar as a soil amendment was more climate beneficial. Others (e.g. Woolf et al. 2014) have come to a similar conclusion, showing that biochar needs to achieve all-source revenue of between $250 to $350 per ton to out-compete fuel use in the absence of a carbon price.

The fact that Airex's new scaled Canadian pyrolysis facility was announced as a biochar facility, but now seems to be producing bio-coal, would seem to support this conclusion.

Relevant Startups

There are many conventionally financed standalone or divisional biochar equipment manufacturers and operators worldwide, but there are also venture financed startups - many of them at an early stage. The more prominent venture backed companies include:

NetZero (France) has specialized in coffee bean waste streams and is operating a 2,000 ton facility in Cameroon and two 4,000 ton facilities in Brazil - co-located with coffee processors. The biochar is sold back to suppliers to allow them to reduce fertilizer use, improve soil health and increase yields. The facilities generate electricity as a coproduct.

MASH Makes (Denmark) is co-locating biochar plants with agricultural waste processing plants in India.

Carbo Culture (Finland) is developing a novel pressurized pyrolysis chamber with syngas as its co-product. Its first commercial scale reactor is planned to go live in 2025 with 7,500 ton annual capacity.

Applied Carbon (US) has developed a field-deployable mobile pyrolysis unit that can produce and spread biochar on-the-fly using fast pyrolysis technology. (Formerly Climate Robotics)

A number of startups are doing biomass-based CDR, but using geologic reservoirs to store the carbon rather than agricultural soils. For example, Charm Industrial (US) which has raised $125M from General Catalyst, has a pyrolysis process optimized for bio-oil (vs biochar) and sequesters the bio-oil by injection into geologic reservoirs. Vaulted Deep is another company that does direct geologic sequestration of waste biomass.

Conclusions

Biochar is an effective long-lived CDR method with production costs that make it an attractive carbon storage technology when it is deployed cost-effectively (co-location/co-processor/co-products). Biochar is also unique in being a non-chemical nitrous oxide inhibitor, and the only one I've found that acts on the denitrification pathway.

In general, biochar production technology seems relatively well specified and mature, although there may be additional scope for improvement, for example to make facilities capable of rapid switchover between feedstocks and co-products.

Rather than production technology, succeeding in the biochar market seems likely to require successfully securing colocation with feedstock partners, and choosing markets where biochar soil emissions and yield benefits are dependable and valuable.

What is still not clear, at least to me, is whether policy should be attempting to explicitly protect the development of the biochar CDR market or let carbon prices and economics dictate whether bio-carbon is used as a fuel, a soil amendment or for other uses like green cement.

Other Articles in this ClimateTech Series

The ClimateTech Market Map

1: Decarbonizing Nitrogen Fertilizer with Electrochemical Processes

2: Anti-Methane Livestock Vaccines

3: Intro to Enhanced Geothermal Energy (Fracked Geothermal)

4: Abating Nitrous Oxide Emissions from Agriculture

5: Decarbonizing Concrete is Hard

6: Decarbonizing Cooking Doesn't Matter (Much)

8: Is There a Path to $1/kg Green Hydrogen?

9: Microbial N Fertilizers Do Not Reduce Emissions

The Climate Change Impacts Series

1: Climate Change Alters Ecosystems

References

(1) Jeffery, S., Abalos, D., Prodana, M., Bastos, A.C., Van Groenigen, J.W., Hungate, B.A. and Verheijen, F., 2017. Biochar boosts tropical but not temperate crop yields. Environmental Research Letters, 12(5), p.053001.

(2) Biochar Zero, 2024. Biochar Project Developer Guide. Available online at: https://biochar-zero.com/free-biochar-project-developer-guide/

(3) https://www.farmersjournal.ie/news/news/biochar-making-300-to-2-000-per-tonne-763428

(4) Karan, S.K., Woolf, D., Azzi, E.S., Sundberg, C. and Wood, S.A., 2023. Potential for biochar carbon sequestration from crop residues: A global spatially explicit assessment. GCB Bioenergy, 15(12), pp.1424-1436.

(5) Lefebvre, D., Fawzy, S., Aquije, C.A., Osman, A.I., Draper, K.T. and Trabold, T.A., 2023. Biomass residue to carbon dioxide removal: quantifying the global impact of biochar. Biochar, 5(1), p.65.

(6) Azzi, E.S., Karltun, E. and Sundberg, C., 2019. Prospective life cycle assessment of large-scale biochar production and use for negative emissions in Stockholm. Environmental Science & Technology, 53(14), pp.8466-8476.

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